Amplifying hearing aid



Aug. 17, .1943. H B, HAPIRO 2,327,320

AMPLIFYING HEARING AID Filed Nokr. 12, 1941 4 Sheets-Sheet 1 "T u INVENTOR HARRYB j/IAP/RO.

g mmulm ATTORNEY 4 SheetsSheet 2 H. B. SHAPIRO Filed Nov. 12, 1941 AMPLIFYING HEARING AID Aug. 17, 1943.

1943- I H. B. SHAPIRO 2,327,320

AMPLIFYING HEARING AID Filed NOV. 12, 1941 4 Sheets-Sheet 3 a E; k LMfij ow 500 1000 1500 2000 zsoo .1000 3500 4000 4500 FREQUENCY CYCLES/SECOND INVENTDR HARRYB. 5HAP/R0 ATTO R N EY Aug, '17, 1943. H. s. SHAPIRO AMPLIFYING HEARING AID Filed Nov. 12, 1941 4 Sheets-Sheet 4 13 INVENTOR HARR Y B. SHAPIR O. BY 3 WWW ATTORN EY keeping the electron cessoriesv required I light and small spicuously worn on Patented Au 17, 1943 UNIT-ED STATES PATENT OFFICE Harry B. Shapiro,

Sonotone Corporation, poration of New york Application November 12,

6 Claims.

This invention relates to amplifying hearing aids, and it has among its objects novel electron tube amplifier hearing aid arrangements and methods that make it possible to supply the user with amplified sound required to compensate for his lack of normal hearing acuity, while tube amplifier and the acior its operation extremely so that the complete electron tube hearing aid may be comfortably and inconthe body of the user.

' The foregoing and other objects of the invention will be best understood from the following description of exemplifications of the invention, referencebeing had to the accompanying drawings, wherein Fig. 1 is an elevational view of the hearing aid amplifier unit exemplifying the invention with the cover, microphone and wiring removed from their locations;

Fig. 2 is a vertical cross-sectional view along line 2-2 of Fig. 1 with the microphone in its location;

Fig. -3 isavertical cross-sectional view of the amplifier chassis along line 3-3 of Fig. 1;

Fig. 4 is a horizontal sectional view of the upper portion of the amplifierunit alongline 4-4 of Fig. 1;

Fig. 5 is a circuit diagram of acomplete electron tube amplifier hearing aid exemplifying the invention;

Fig. 6 is a vertical sectional view through the connector plug for the amplifier unit of Fig. 1 and the associated portions of the amplifier chassis;

Fig. '7 is a cross-sectional view along line 1-1 of Fig. 6; y 7

Fig. 8 is a cross-sectional view along line 8-8 of Fi .6;

Fig. 9 is an enlarged horizontal view along line 9--9,oi Fig. 6;

.Figs. 10 to 13 are explanatory curve diagrams;

Fig. 14 is a diagrammatic view similar to Fig.

'5, illustrating a modified hearing aid amplifier arrangement of the invention;. Fig. 15 is a view similar to Fig. 5 of another exemplification of the invention;

Fig. 16 is a vertical cross-sectionalview along line l6-l6 of Fig. 15;

Fig. 1'7 is a'vertical cross-sectional view alon line l'l-l1 of Fig. 15; and

Fig. 18 is a view similar amplifying the invention.

Most .of the hard of hearing or deaiened per- Cleveland, Ohio, assignor to Eimslord, N. Y., a cor 1941, Serial No. 418,857

. hearing impairment.

fortably and inconspicuously on the body of the user. In addition, it must be simple and tool proof in operation, and it should require little attention and a minimum oi parts that may get to Fig. 5 illustrating another hearing aid amplifier arrangement exout of order or require replacement, so as to free the user from mental as well as physical strains.

Various efforts have been made in the past to devise an electron-tube amplifier; hearing aid meeting these requirements. Since such hearing aid electron-tube amplifier together with its batteries and accessories must be small enough for comfortable and-inconspicuous wear on the body of the user. it must be designed so that all its elements may be ing having overall dimensions not larger than about 6 xiv/ x 1 inches, fitting into a pocket of the user. In addition, the overall current drain required for its. operation must be kept small ly fiat over the principal speeh frequency range between about 500 and 4000 cycles persecond, and itv must be designed to deliver to the receiver an output of about 10 miiliwatts or more whendriven by the voltage of a self-generating microphone, such as a Rochelle crystal mechano-electric microphone transducer, giving an output of about 5 to -10 millivolts when actuated by speech of normal loudness from a distance of about one foot.

A 10 'miliiwatt output may be obtained from a power amplifier stage using one of the best types of midget power-amplifier electron tubes by impressing on it an input voltage of about 2 volts.

7 Such power amplifier tube requires a load impedance of only about 20,000 to 200,000 ohms, 'and because of its relatively low impedance, the design of a midget size load transformer or choke coil required for coupling the receiver load to the plate circuit of the power amplifier'tube presented no superable problem.

In order to supply to the power amplifier stage an input voltage of about 2 volts, such electron tube hearing aid amplifier must have a voltage and seek to conceal their housed within a flat casspaced electrodes amplifier section giving a gain greater than 100, and preferably at least 200.

Prior to the invention it was considered impossible to obtain with a single voltage amplifier stage operating with plate battery voltage of the order of 45 volts a gain of more than 50. Accordingly, at least two voltage-amplification stages were considered essential in order to obtain the voltage needed to drive the power amplifier tube. As a result, all prior electron tube hearing aids followed the common and successful practice evolved after long development work in the audio-frequency amplifier field, and used at least two cascaded resistance-coupled voltage amplifier tubes for securing the required voltsize and weight of such multi-stage resistance coupled amplifier unit within the required limits. Such resistance-coupled amplifier arrangement had also the additional advantage of uniform amplification over the principal parts of the speech frequency range.

As distinguished therefrom, the more complicated inductively coupled voltage amplifier stages, that were used in conjunction with triode amplifier tubes before the advent of the screen and pentode tubes rendered them obsolete in the audio-frequency field,'were considered impossible for a wearable hearing aid not only because of their peaked output, but also because, in order to prevent saturation by the D. C. plate current, they required relatively bulky magnetic cores, and because the relatively large distributed capacity of their windings limited their maximum gain.

Very serious difficulties have also been encountered in manufacturing midget electron amplifier tubes of sufficiently uniform gain and other important operating characteristics required for such hearing aids. This is due to the fact that the operating characteristics of such multi-electrode tubes are determined'by the ratios of the distances between the electrodes and their areas and also by the condition of the electrode surfaces and of the vacuum. In midget size electron amplifier tubes, the maintenance of the ratios of the distances between the extremely closely required for tubes of sufficiently uniform operative characteristics is very difficult, and presents a critical problem.

As a result of extended experimentation, I have found that there exists a critical range of operating conditions under which a screen grid electron amplifier tube operating in conjunction with a midget size coupling inductance having a highpermeability core inductance of a cross section of about 1 6 of a square inch or less, will operate as an inductively coupled voltage amplification stage with a gain of more than 100 over the frequency range between 500 and 4000 cycles and a gain of more than 200 over a somewhat more limited part of the principal speech frequency range; and that such single inductively coupled voltage amplification stage operated within such critical range may be combined with only one additional power amplifier stage into a small fiat amplifier unit fitting the vest pocket of the user and giving with a microphone input of only about to 10 millivolts, an output of about 10 milliwatts having an overall frequency response of the type equired by a good hearing aid. The resulting elimination of the additional electron tube amphfier stage is of great value, not only because it replifies the assembly, service and maintenance of the amplifier, and because it reduces the items I gions in which the hearing impairment of the {with the individual needs of which user would make it objectionable, while securing the high amplification in the frequency regions as required by the character of the hearing impairment of the individual.

In accordance with theinvention, such selectively controllable electron tube hearing aid is obtained by combining the inductive impedance of the receiver forming a part of the output load with a shunting circuit including a-condenser element, the effectiveness of which is arranged to be controlled so as to either maintain a substantially uniform response over the entire princlpal speech frequency range or to selectively increase or decrease the response in accordance the deafened person.

Furthermore, the amplifier arrangement of the invention makes possible such selective control of the amplification in the different parts of the speech frequency range without substantially reducing the overall volume of the output.

Before proceeding with an explanation of the principles underlying the invention,"I shall first describe one specific embodiment thereof in the form of a-complete electron tube hearing aid shown structurally in Figs. 1 to 4 and diagrammatically in Fig. 5. The complete hearing aid comprises a microphone 20, an electron tube amplifier having a single voltage amplifier tube 2|,

a power amplifier tube 22 and the associated circuit elements, all housed in a compact flat casing 23 which is small enough for inconspicuous wear,

for instance, in the vest pocket of the user. An electrical battery assembly 24 small enough to be inconspicuously bias cells 26 and a 21 is connected to from 5 to 10 millivolts when actuated by speech of normal loudness from a distance of about one foot. Alternatively, a condenser microphone,

is combined with an electret so as to generate voltages in response to the vibrations imparted to one of the condenser microphone elec trodes, may be used. The amplifier is designed for use with any standard electromagnetic or piezo-electric electro-mechanical receiver transducer now on the market. Standard bone conduction receivers of the type described in Re. Patent 21,030, which have a resistance of about 20 ohms and an inductance of about 10 millihenries and require about 15 to 20 milliwatts driving energy, perform excellently with such amplifier. The electron tubes 2|, 2.2 are commercially available midget pentodes having an outer diameter of about I; of an inch, and a height of about duces the battery drain, but also because it sim- '76 1% inches. each tube having ananodo or plate worn by the user, comprising -a battery unit formed of a multi-cell anode or plate 2,327,320 3 3|, a suppressor grid 32, a screen or regulating amplifier housed within the casing are secured grid 33, a control grid 34 and a filamentary cathto a substantially rigid plate-shaped chassis ode 35, to which the suppressor grid 32 is conframe 63 of insulating material, such as a textile nected. Such pentode tubes havean amplificafabric lamination impregnated and solidified by tion factor in excess of 300 and are able to give 5 an insulating synthetic resin. The coupling inan output ofabout 14 milliwatts when operated ductance 42 and the transformer 52 are located with a plate supply of 45 volts and a heating cur-- within suitable cutoutsof the fram 63 and held rent of about 50 milliamperes at a .75 to 1.5 filain place by'looped clamping wires 54 fastened to merit voltage. The two cathodes 350i the tubes the frame 63. The amplifier tubes 2| and 22 are 2|, 22 are connected through leads 36 and 31 to located within suitable cutouts in the frame 63 the amplifier terminals +A, A, of the amplifier and are held in place by clamping loops 65 and casing to which the cathode heating cell 21 is consupporting brackets 66 having grommets 66a of nested, the cathode heating circuit being comcushioning material, such as rubber, within which pleted by the common grounded return lead 38 the tips of the amplifier tubes 2|, 22 are held.

and the three-contact switch 39, which is shown 1 The volume control 4| and the response conin Fig. 1 in the open position. v trol rheostat 54 are enclosed in fiat circular The microphone 2B is connected through a volmetallic casings 51 held clamped within cutouts of ume-control potentiometer 4| having a resistance the frame 63 and are provided with circular cover of about a megohm to the control grid 34 of the plates 'IO, which project through slits in the side voltage amplifier tube 2|, the other side of the walls of'the casing 23 and serve to actuate the microphone being connected to the grounded lead moving contact elements of the rheostats.

38 from the --A terminal. The plate 3| of the The condenser elements 45, 46, 53 of the amvoltage amplifier tube 2| is connected through a plifier are likewise located within suitable cutmagnetic-core coupling inductance 42, by way outs of the frame 63 and are secured thereto by of lead 43 to the terminal +B of the amplifier suitable strips which are fastened to the clampcasing, to which the positive terminal of the plate ing plate. The resistor elements 44, 41 of the supply battery 25 is connected. The screen grid amplifier are likewise located within cutouts of 33 of the voltage amplifier tube 2| is maintained the frame 63.

at the required potential determined, in the way The upper wall portion of the casing 23 is prodescribed hereinafter, by connecting it through a 0 vided with a cutout 13 facing a similar cutout resistor. 44 to the positive plate voltage lead 43, 14 in the frame pl te 53 on the pp r d of and a by-pass condenser 45 of .03 microfarad is which are mounted .the terminal members +B, connected between the screen grid 33 and the A, +A, C, R, RG, of the amplifier unit. The grounded lead 38. terminal members are provided with plug and The amplified voltage developed across th 011- socket portions arranged for contact engagepling inductance 42 is impressed through a 6011- ments with cooperating terminal members of a pling condenser 46 upon control grid 34 of power p 15 provided at the end f the co 8 f om amplifier tube 22, the control grid 34 being com the battery which fits within the space of the cutnected through a grid leak resistor 41, f 5 out I3 provided in the top wall of the casing. megohms, and a lead 48 to the grid bias ter- The p 15 s s w in d l in Fi s. 6 to 9 minal C of the amplifier casing 23 t which th and it has, in addition to the socket terminal negative terminal of the bias battery 26 is 0011- members making contact engagement with the nected so as to apply the required negative bias p p terminal members C potential to the control grid of the power ampliof the amplifier unit, also two p s terminals 16 fie t b 22, Th plate 3| of t power amplifier which, as shown in Fig. 8, are provided with socket tube 22 is connected to the primary winding 5| of e o s 1 shaped for engagement with the a coupling tran former 52, th econdar windterminal tips 18 provided at the ends of the 00rd ing of which is connected to the receiver terleads 30 from the receiver. This arrangement minals R, RG of th amplifier casing, to which makes it possible to use a singleplug 15 for dis the receiver cord 30 and the receiver 29 are con- 5 connecting the ampl fi from the cord 28 leading nected. Parallel to the primary winding 5| of $0 the battery terminals and the c d 30 leading the output transformer 52 is connected a conthe receiver, While Permitting independent r denser 53 and a serially. connected rh t t 54, moval of the receiver cords 30 from the plug memwhich, as will be explained hereinafter, serves her 15 While it remains i ts normal Operating to control the frequency response of the amplifier. 55 Position in which it connects the battery to the The output transformer 54 is designed to couple amplifier circuits. This arran ent enables the receiver to the output circuit of the power quick xchange or r placement of the receiver, amplifier tube 22 and has a transformer ratio for instancerby Substituting an a conduction of t, 1 25 I ceiver for a bone conduction receiver, or vice All the elements of the electron tube amplifier, G verse, without removing t p g 5 with its including the microphone in, are housed Within y termirgals from ts p mg p s t in he the small fiat casing 23; which is shown in acamplifier tual size in Figs. 1 to 4 of the d awings and has The switch 39 has an ar y p i t ne coverall dimensions of only about 4 x 2 /4 X mating knob 53 and s ed in 9/ e u r inches. The output-transformer 54 has amagmetallic casing 59 having P OJ Q s d rackets 'netic core with a cross section of 1 s of an inch. It which are secured to the upp r g portion of has an overall size of the order of a cubic inch the frame 63 so as to hold the switch aligned along comparing in size with a midget tube, and weighs the plug 15 at the upper edge of the casing 23, about one ounce. Thecoupling inductance has with the switch knob 58 projecting outwardly the same dimensions and weight of the output above the plug 15 and-the edge of the casing. transformer, and is only'of about the size of a The ends of the electrode leads of the tubes 2|, midget tube. 22 are soldered to tips of tube connector pins 61 As shown in Figs. 1 to 4, the casing 23 has a riveted to the frame 63 which serve as an addiremovable cover 60 clamped to the casing by tional support for the tubes. The tube connector screws 6|, and all'the operating elements of the pins are provided with transverse holes 68 in All the elements of the amplifier unit are assembled and firmly fastened to the mounting plate 63 and are available for testing before assembly within the casing. The frame 63 with the assembled amplifier elements fits within the shallow casing 23 and is secured to its curved wall shaped to fit against; the, body of the user by means of screws 69. a

The microphone is mounted in the hollow casing space in .front of the part of the frame plate 63 to which are secured the volume control unit 4|, the amplification control rheostat 54, the condenser elements 45, 46, 53, and the rheostat elements 44, 41, which are all fiat and do not protrude. The microphone 20 is of the high-impedance self-generating crystal type, such as described in my copending application Serial,No. 350,595 and Serial No. 394,527, filed August 3, 1940, and May 21, 1941, respectively,-and it has a Rochellecrystal element enclosed in a fiat circular casing 80, the crystal element being driven by a vibratory diaphragm secured behind its dome-shaped ings through which the diaphragm is exposed. The circular microphone casing 80 is held suspended within a collar 8| of elastic material, such as rubber, the circular front edge of which engages and forms an acoustic seal with the facing peripheral portion of the casing cover 60 surrounding its sound pervious wall portion 85. The rearward portion of the elastic collar 8| rests against the underlying portion of the frame plate 83 and has holes 82.

The sound pervious wall portion 85 of the amplifier casing cover 60 facing the diaphragm of the microphone 20 is recessed in the way shown in Figs. 1 and 4, and provided with the transverse openings 86 arranged so that the sound waves in the surrounding air shall actuate the microphone. By providing the casing holes 86 through which the sound is propagated to the microphone in the recessed wall portion 85, the part of the dress, of the ing is worn user, under which the amplifier casis kept away from the edges of the casing holes 86 through which the sound is propagated to the microphone. This arrangement prevents the generation of noise by the rubbing of a garment portion against the edges of the casing holes 86 opening into the microphone, and is instrumental in eliminating disturbing garment rubbing-noises that would otherwise be propagated to the microphone and be delivered in amplified form to the receiver.

The interior walls of the casing 53 and its cover are covered with a conducting coating of metal or graphite, for instance, so as to shield the amplifier circuits. The exposed conducting parts of the amplifier um't mounted on the frame 63 are separated from the casing wall by a layer of insulating material. The leads extending between the successive amplifier stages of the amplifier unit are shielded against each other by interposing between them thin sheets of conducting material which are allconnected to the ground, shown in Fig. 5 by the grounded lead of the amplifier unit which is connected to the A terminal of the battery. The metal casings 51 of the volume control 4| and the frequency control rheostat 54 and the metal casing 59 of the switch 39 are likewise interconnected with the ground. To assure a good ground connecmicrophone cover 84 having open- 7 garment, such as the vest pocket or generating microphone tion between the elements of the amplifier unit and the shielding inner walls of the casing, the grounded casing 59 of the switch unit 39 and the grounded casing 51 of the rheostat 54 are provided with outwardly biased spring contacts 88,

89, respectively, arranged to establish contact with the conducting interior walls of the casing when the amplifier unit is assembled within the casing.

In order to secure with a crystal-type selfand a two-tube amplifier hearing aid arrangement described above in connection with Figs. 1 to 5 an output of more than ten milliwatts required to drive a standard bone conduction receiver with a good frequency response in the range between 500 and 4000 cycles, required in a satisfactory electron tube hearing aid, the voltage amplifier stage associated with the amplifier tube 2| must be able to give a gain of 200 and a good frequency response in the range between 500 and 4000 cycles varying by not more than plus or minus 5-decibels.

In order to obtain with a single inductively coupled voltage amplification stage a gain of more than and a good frequency response in the range between 500 and 4000 cycles per second, a coupling inductance that has an impedance of more than about one megohm in the range be tween 500 and 4000 cycles is required. Those who had familiarity with the inductances used in prior art inductively or transformer coupled audio-frequency voltage amplifier stages, that were used with triode tube amplifiers before the much higher amplification of pentode tubes made it possible to obtain better meability magnetic core laminations by the D. C. component of the plate current, it would be necessary to use a large magnetic core structure of great cross section that could not be placed with- In the hearing aid amplifier arrangement described in connection with Figs.

' more than 200 with a good frequency rein the range between 500 and 4000 cycles, required for a satisfactory electron-tube hearing aid, is made possible, with a single voltage amplifier stage associated with the amplifier tube 2|, by the discovery that a midget size inductance formed of high permeability magnetic core laminations having a cross section of less than I; of one square inch and an ordinary winding occupying not more bulk than the core structure, when used as a coupling inductance for a distributed capacity of coupling inductance, and the input mains above one megohm screen grid electron tube operated with a plate battery voltage of .the order of 45 to '75 volts or even less, will, under certain critical conditions, operate with an impedance of more than about one megohm in the frequency range between 500 and 4000 cycles, and an impedance of several megohms in a somewhat ductively coupled amplifierstage will, under special critical operating conditions, give a gain as highas 200 and more with a satisfactory frequency response in the principal speech frequency range required for a tube amplifier hearing aid. 1

The critical conditions which have to be observed in devising such inductively coupled midget voltage amplifier stage will now be explained by a reference to the curve diagrams of Figs. to 13.

In Fig. 10] the curve RP gives the plate resistance, and the curve IP gives the plate current of the pentode 2| as a function of the screenvoltage, when the screen voltage is increased from 45 volts while the plate 3| is connected to a 45 volt battery. In Fig. 11, curves LA and LM give the inductance of the midget size coupling inductance 42 shown in Figs. 2 tot as a function of the current flowing through its windin when using magnetic core laminations of high permeability magnetic material known commercially as- MU.

and Allegheny metal, respectively. In Fig. 10, corresponding curves LM and LA have been plotted from Fig. 11 to indicate by their ordinates the values of these inductances for different screen voltages of tube 2 l.

In the audio-frequency operating range of midget type screen-grid or pentode amplifier tubes, the amplification factor may be considered as substantially constant and the voltage gain of an amplifier stage using such screen-grid or pentode tubes may be considered as substantially proportional to the quotient of the coupling 1m.- pedance over the sum of the plate resistance and the coupling impedance. The coupling imped ance in turn is determined by the value of the inductance L of the coupling coil and by the effect of the capacitive shunt impedances, namely,

the inductance winding, the wiring connected to the capacity of the stray capacity of the following amplifier stage.

As shown in Fig. 10, by the curve IP, a decrease of the screen voltag of tube 2| below the value of the plate voltage decreases the plate current narrower part of the speech frequency range; and that such inthe plate current of the tube is adjusted within a critical range of values at which the coupling inductance forms with the associated capacitive shunt impedances a parallel resonant circuit having a resonant frequency in the range between about 900 and 4000 cycles; and that under such conditions, the amplifier stage will operate with a gain of as much as 200 over the parts of the speech frequency range important for an electron tube hearing aid amplifier.

These conditions are illustratedby the frequency response curves LMIO to LMMI and LAIO to LAN of Fig. 12, the ordinates of which give the relative values of the voltage output across a resistor load connected to the secondary winding of the output transformer 52 when a constant input voltage of varying frequency control grid 34 of the voltage amplifier tube 2|. For every practical purpose, these curves LMIO ,LAlll of Fig. 12 also represent the frequency response of the voltage developed across the coupling inductance 42, except for the fact that the actual voltage gain developed across the coupling inductance 42 in the range below 1000 cycles is slightly higher than shown in the curves of Fig. 12.

The ordinates of curves LMIO to LM40 give the voltage gain across the coupling inductance 42 made of MU metal when the screen grid 33 of tube 2| is maintained at 10, 15, 20, 25, 30, and volts, respectively, and the ordinates of curves LAN! to LAN) give corresponding gain values for E, and as shown by curves RP, LM, LA, a decrease of the plate current is accompanied'by an 4 increase in the plate resistanceRP and anincrease in the value of the inductance of the coupling inductance 42. Since the value of the coupling inductance determines the value of the efiective coupling impedance, thesevariations of the magnitude of the plate resistance and ofthe coupling inductance, determine the magnitude .and the character of the gain. at the different plate currents, corresponding to the diiferentscreen voltages- I have found that by making the coupling inductance' or screen-grid of pentode tube voltage amplifier stage with a midgetsizehigh permeability core having a cross section of less than 1 of a square inch and placing thereon sufficient turns of an ordinary winding to obtain an impedance of more than one me'gohm at 600 cycles, the winding of such inductance has a sufilciently low distributed capacity sothat its impedance reup to 4000-cycles if of about 225 the average gain a coupling inductance of Allegheny metal.

As shown by curves LMIO and LAN of Fig. 12, when using coupling inductances having cores of either MU metal or Allegheny meta1, at screen voltages lower than 10 volts, corresponding to a plate current of less than about lo nicroamperes, in the range between 500 and 4000 cycles is less than 100. This small gain, at screen voltages which keep the plate low 10 microamperes, results from the fact that, as shown in Fig. 10, the plate resistance at such low platecurrents rises at a rate so much higher than the coupling inductance that, notwithstanding the fact. that for such small plate currents the value of the coupling inductance is at -its maximum, the plate resistance that it-reduces the gain down to below 100.

As shown by curves LMI5 and LAI5, when using coupling inductances of MU metal or Allegheny'metal, at screen voltages of about 15 becomes .so large volts, corresponding to the plate currents in the gain rises rapidly in the low frequency range, remaining' above 200 in the I frequency range between about 500 and 2500 cycles, the peak of the gain being inthe range of about 1000 cycles.

As shown by curves LM25 to LM30, and LAZS to LA", when using coupling inductances of MU metal, at screen voltages volts, corresponding to plate currents in the range between about 225 to Y 350 ymicroamperes, and

when using coupling inductances of Allegheny metal, at screen voltages between about 25 and 40 volts corresponding to plate currents in therange to .500 microamperes, the voltage is applied to the current bemin and man, when betweenabout 25 and 30 I gain rises rapidly with the rise in the frequency remaining substantially above 200 in the frequency range between 500 and 4000 cycles, the peak of the gain being in the range between about 1000 cycles and 2000 cycles.

As-shown by .curves LM35 and LM40, when using coupling inductances of MU metal, at screen voltages of 35 and 40 volts, corresponding to plate currents in the range of 500 microamperes up to about 2 milliamperes, the gain rises in thelow frequency range till it becomes about 100 at about 1000 cycles and it rises above 200 in the higher frequency range above about 2000 cycles, the peak of the output being in the range above about 3000 cycles.

The foregoing analysis of the operation of such inductively coupled voltage amplifier stage described in connection with Figs. 1 to 6 shows that in order to obtain a gain of more than 100, the

plate current of a screen grid or pentode tube must be maintained sufliciently large and greater than about 10 microamperes so as to assure that the plate resistance is not excessive and does not reduce the gain below 100, although the inductance of such midget size coupling inductance rises to a maximum value with a decrease of the plate current. In order to secure a gain of more than 100, the plate current pentode tube has to be at least 10 microamperes, the gain rising and becoming a maximum in the frequency range between about 1000 and 2000 cycles when the plate current is increased to a .value in the range'between about 200 and 300 microamperes. A further increase of the plate current results in a decrease of the gain because as shown in Fig. 8, the saturating effect of the D. 0. plate current reduces the effective inductance of the m dget coupling inductance 42, and when the plate current is increased above 500 microamperes up to about one or'two milliamperes, the gain rises with the frequency in the range up to about 2000, reaching a gain of about 100 at about 1000 cycles and about 200 at about 2000 cycles, and remaining above 200 between about 2000 cycles and 6000 cycles, the peak of the gain being in the range higher than about 3000 cycles.

The high gain is secured with such inductively coupled voltage amplifier stage notwithstanding the presence of a grid leak resistor 47 in the following amplifier stage, by making the grid leak resistor 41 so high that it does not materially shunt the coupling inductance 42. In the amplifier of Figs. 1 to 6, this result is attained by making the grid leak resistor megohms.

I have found that the foregoing characteristics render such inductively coupled midget size voltage amplifier stage operating from a low plate battery ideal for an electron tube hearing aid amplifier and makesjt possible to produce with only one additional power amplifier stage a compact fiat electron tube amplifier unit of the size of a cigarette case, which isable to deliver a power output that is substantially flat within plus and minus 5 decibels over the principal speech frequency range between about 500 cycles to 4000 cycles, and which may also be controlled to selectively increase and decrease the amplification in the different parts of the speech frequency range in accordance with the hearing impairment of the user.

I have also found that the operating conditions secured by an inductively coupled amplifier stage described above in connection with the curves of Figs. to 12 are very efiectivein com of the screen grid or I the frequency range is to be out off,

stantially flat within plus or minus 5 pensating for relatively large variations in the operating characteristics of midget electron amplifier tubes. .As a result, such inductively cou pled amplifier stages may be equipped with multi-electrode amplifier tubes of a much lower degree of uniformity than feasible with resistance coupled amplifier stages, and the problem of manufacturing satisfactory midget amplifier tubes for such application is rendered much less difllcult.

The combination of such inductively coupled voltage amplification stage operating under the conditions corresponding to the curves LM25 to LM30 and LA25 to LA35 in Fig. 12 with a power amplifier stage in the way described in connection with Figs. 1 to 6 will give an amplified power output which is substantially uniform within plus and minus 5 decibels over the frequency range between 500 to 5000 cycles.

only one voltage amplifier stage with one power amplifier stage in the Way shown in Figs. 1 to 6 are illustrated in Fig. 13 in which curve OVR, corresponding to the curve LM25 in Fig. 12, shows the frequency response characteristics of the output developed across the coupling inductance 42 of the amplifier tube 2| when a constant voltage input is applied to the control grid of tube 2| and the screen grid voltage is maintained at 25 volts. By combining such inductively coupled voltage amplification stage with a power amplifier tube 22, which delivers its output to an inductive load formed by the receiver 29, the overall response of the output delivered to the receiver assumes the form indicated in Fig. 13 by curve 0V.

In the specific case described hereinabove, the inductive load was formed by a standard commercially available hearing aid receiver having an inductance of about 10 millihenries connected to the plate circuit of the power amplifier tube 22 through an output transformer having a ratio of primary to secondary windings of 25 to 1. shown by the curve 0V, the power amplifier stage operating with such inductive load levels the peaked response characteristics of the voltage amplification stage by raising the high frequency range and slightly lowering the low frequency range. Although, as shown by curve 0V in Fig. 13, the overall frequency response drops in the low frequency region, this effect is compensated by the fact that the impedance of the receiver also drops in the low frequency range. As a result, the output delivered by'the receiver is subdecibels over the frequency range between about 400 to 5000 cycles.

The crystal'element of the-microphone 20 constitutes a capacitive impedance which is at a maximum at low frequencies and falls off as the frequency increases from its lowest value. This response characteristic of the microphone automatically raises the lower part of the overall response characteristics of the hearing aid amplifier combination, described above, as represented by the curves of Fig. 13. By properly proportioning the shunting resistance connected parallel to the microphone 20, the response in the lower part of the frequency range may be proportioned to have the desired value. For instance, in cases where the response in the lower part of the shunting resistance 44 connected parallel to the microphone 20 is made sufliciently low compared with slope of the rising low frequency part of ,.indicated by curves LM35, LA35,

the capacitive impedance of its crystal element e as to assure that at all frequencies below a selected low frequency, for instance, 500 cycles, the voltage developed by the microphone across the resistance 4| may be made only a fraction of the voltage which a. sound input of a given level will develop across the resistance 4| at frequencies above the selected frequency.

The amplifier arrangement of the invention described above makes it also possible to regulate the electron, discharge conditions in the voltage amplifier stage of tube 2| within the critical operating range in which therequired high gain is obtained by adjusting the value of the plate current through the regulation of the screen grid voltage so as to place the peak of the voltage output of the voltage amplification stage in a selected 'part of the frequency range and thereby secure in. conjunction with the power amplification stage an overall amplification which has the desired selective frequency response needed in order to compensate for the different specific cases of hearing impairment.

Thus, as shown by curves LMIU, LA|

,LM40 of Fig. 12, the maximum overall gain may be increased or in general modified by adjusting the voltage applied to the regulating or screen grid electrode 33 to maintain the plate current at the desired value within the critical range explained hereinabove. This regulation of the plate current by adjustment of the screen voltage of the voltage amplifiertube 2| modifies not only the maximum gain of the voltage amplifier stage but also the slope of the rising low frequency part of the output characteristic below the peak frequency. The fiat maximum gain part of the overall output characteristic is determined by the frequency peaks corresponding to the different operating conditions as determined by the different screen voltages applied to the voltage amplifier tube 2|.

and the corresponding D. C. plate current. The character of the rising slope of the of the voltage amplifier, so as overall. output characteristic of the power amplifier stage as well as the fiat maximum gain part may thus be readily controlled;

by regulating the screen voltage and therethrough the plate current of the voltage amplifier tube 2|. I

As explained in connection with Fig, 12, the regulation of the plate current by adjusting the screen voltage modifies not only the maximum gain of the voltage amplifier stage-but, also the the output characteristic in the way shown in curves LM|5 to LM40, LAW-to LA40. Accordingly, by proper adjustment of the screen voltage and therethrough of the plate current, of the voltage amplifier tube 2 i, the character of the rising slope of the overall output characteristic of. the power amplifier stage as well as its fiat part in the frequency range up to 5000 cycles may be modified in accordance with the needs of the individual. For

instance, by adjustment of the screen voltage of the voltage amplifier tube 2| of 35 volts, the low frequency response may be reduced in the way to a value desirable-for types of deafness which require reduced low frequency response and a larger gain in the high frequency range.

cult of the power amplifier quency response of the f For instance, if the rheostat 54 is in a position in which the full resistance of one megohrn of in series with the tuning condenser 53, the output response will of more than and a frequencyresponse desirable for a hearing aid amplifier in the way described hereinabove is obtained if the plate current is kept in the range between about 10 microamperes and about 2-milliamperes.

A distinct feature of the invention resides in the utilization of the inductive load formed by the receiver-for selectively controlling the amplification in the different parts of the frequency rang in accordance with the needs of the user, this phase of the invention being applicable to electron tube hearing aids using-resistance coupled voltage amplifier stages or only one inductively coupled voltage amplification stage as'described herein.

In accordance with the invention, 1 take advantage of the inductive load formed by the hearing aid receiver connected to the power amplifier stage by combining it with a parallel connected condenser'so as to form in the plate cirstage a tuned parallel resonant load impedance circuit which resonates at the low frequency at which a high amplification is required, so as to selectively boost the desired part of the low frequency response and reduce the response in the other parts of the irequency range in accordance with the needs of the individual.

In the arrangement of the invention-described.

' tiveness of which is controlled by the rheostat 54 connected in series with the condenser 53 so as'to selectively determine the character of the overall output characteristics of the amplifier.

In the specific form of amplifierarrangement. described in connection with Figs. 1 to 5, the receiver 29 having an inductance of about 10 millihenries, of the power amplifier tube 22 through a transformer having a transformer ratio of 1 to 25, will resonate at about 1000 cycles if a capacity 53 of .003 microfarad is connected in parallel to the primary winding 5| of the inductance. By using a rheostat 54 adjustable from a maximum of 1 megohn to zero, the efiectiveness of the condenser 53 in modifying the character of the overall output delivered to the receiver may be selectively controlled .to adjustably modify the freoutput.

the rheostat is connected be unaffected by the condenser, and willremain substantially of the form given by curve 0V in Fig. 13, as explained hereinabove;

The curves OYX and DVD! show the modified character of the overall response of the amplifier obtained by successively reducing the resistance of the rheostat 54 to 25,000 and 10,000 ohms, respectively, showing the efiectivene ss of this selective control of the overall response of the amplifier. The tuning condenser 53 with the ,aid amplifier is obtained when the screen voltage the inductance of the rewhich is connected to the plate circuit in the high frequency range than V quency range; and when such receiver is actucontrol rheostat 54 serves thus as a very simple and effective control of the character of the output and enables ready adjustment of the overall amplification characteristics of the amplifier so that they meet the individual requirements of the user.

As shown by the curves OVX and OV2X of Fig. 13, this selective control of the response described herein is effected without substantially reducing the overall volume of the output, because the lowering of the response in the high frequency region is accompanied by a simultaneous increase of the output in the low frequency region, keeping the overall volume of the output substantially unchanged.

' The control of the frequency response of the output by combining an inductive receiver load connected to the power amplifier stage with parallel connected capacitance into a resonant circuit having a resonance frequency in a selected part of the speech frequency range and means for adjustably limiting the effectiveness of the capacitance of the type described above. may be used with other types of hearing aid amplifier arrangements if the receiver forms an inductive load having an inductance of 10 microhenries or more.

Furthermore, the inductance of the output load formed by the receiver may be combined with a condenser arrangement by proper correlation of the transformer ratio of the output transformer with the characteristics of the receiver, in the way described above, to increase the. amplification in any selected parts of the speech frequency range so as to compensate for different types of hearing impairment.

Where a wider range of control of the output is desired, it may be obtained by varying the voltage applied to the screen electrode 33 of the voltage amplifier tube 2| so as to shift the peak of the output of the voltage amplifier stage to the low or to. the high frequency range to either produce a dip in the amplification at an intermediate frequency, or to selectively boost the high frequencies or the low frequencies, or to give the amplifier a uniform response over substantially the entire speech frequency range.

A hearing aid amplifier arrangement of the type described in connection with Fig. will also operate satisfactorily if it is used in conjunction with a piezo-electric receiver instead of with a magnetic type of receiver indicated at 29. Such piezo-electric receiver when connected to the plate circuit of a power amplifibntube represents a capacitive load and the available piezoelectric receivers have an effective capacity of about .0015 to .002 microfarad. When such capacitive load is connected to the plate circuit of the power amplifier stage of an amplifier of the type described in connection'fwith Fig. 5, the out put voltage impressed upon the receiveryvill have a drooping frequency response indicated by curve OVC in Fig. 13. However, the available piezoelectric receivers are designed'to be more eificient in the low freated by a voltage of the type shown by curve OVC in Fig. 13, the rising frequency characteristic of such piezo-electric receivers will compensate for the drooping voltage, nd the sound output of the receiver will be substantially fiat over the speech frequency.

An arrangement for enabling interchangeable Ilse of a commercially available piezo-electric redeiver 90 or an electromagnetic receiver 29 in connection with a hearing aid amplifier of the type described above is illustrated in conjunction with Fig. 5, by connecting such piezo-electric receiver parallel to the primary winding 5| of the output transformer 52 while the secondary transformer winding 52 remains open.

Since the primary winding 5| of the transformer 52 has a high inductance of about 50 henries, its impedance is about 150,000 ohms at 500 cycles. As a result, the output would be greatly raised in the low frequency range. In order to improve the response, an additional resistor 9| is connected in shunt to the primary transformer winding 5|. By making the shunting resistance of the resistor 9| equal to about equal or two or three times the capacitive impedance of the crystal receiver at about the highest frequency at which amplification is desired, the amplifier of Fig. 5 operates as a resistance loaded amplifier with a response characteristic of the type shown by curve OVR in Fig. 13. Since, as pointed out above, commercial piezo-electric receivers are more efficient in the high frequency range than in the low frequency range, such receiver, when actuated with an output having a response char-- acteristic of the type shown by curve OVR in Fig. 13, will deliver a sound output having a substantially fiat frequency response which does not vary by more than plus or minus 3 decibels over the range between 500 and 4000 cycles.

As shown in Fig. 5, the additional shunting resistor 9| is connected between the end of the primary transformer winding 54 leading to the plate battery and a spring contact 92 which is mounted with the cooperating switch contact 93 at a jack 94 located adjacent the receiver terminal R of the amplifier unit, the contact member 93 being connected to the plate of the amplifier tube 22, so that by closing the switch contacts 92, 93 through the insertion of a jack plug into the jack 94, the resistor 9| is connected parallel to the primary winding 5| of the transformer 52. The piezo-electric receiver has its two leads provided with a longer plug pin 95 for insertion into the jack 94 and a short plug pin 96 for insertion-into the socket R,.and thereby connecting the piezo-electric receiver 90 across the resistor 9| in the plate circuit of amplifier tube 22 of the amplifier unit, the end of the resistor 9| leading to the +B terminal of the amplifier unit being amplifier.

lead plug 95 of the piezo-electric receiver 90 is removed from the jack 94.

Although in the exemplification of the invention described in connection with Figs. 1 to 5, pentode amplifier tubes are used in the voltage amplification and power amplification stages, other types of amplifier tubes having similar operating char-e acteristics may be used. Instead of using a, coupling inductance of the type described in connection with Figs. 1 to 5, an auto-transformer may be used under certain conditions for a voltage amplification stage in accordance with the principles of the invention. Such an arrangement may be required in case the input circuit elements of the following amplifier stage act as a shunting capacity for the couplinginductance which makes the resonant frequency of the coupling impedance of the voltage amplification stage too high and outr 2,827,820 .tion with Fig. 12. By using the transformer action of the additional secondary turns of an auto-transformer coupling inductance to reflect a larger capacity into the primary side of the autotransformer, the resonant frequency of the resonant coupling circuit impedance is thereby lowered to the required part of the speech frequency range. I

Fig. 14 illustrates diagrammatically an exemplification of an amplifier arrangement of the invention which is similar to that described in connection with Fig. 5 and which uses an autotransformer J as a coupling inductance for the voltage amplification tube 2|. The auto-transformer N10 has an intermediate tap |0| connected to the plate 3| of the voltage amplifier tube 2| so as to include in the plate circuit only sufficient turns to secure the required gain which is stepped up by the additional transformer turns I02 of the auto-transformer and impressed through the coupling condenser 46 on the control grid 34 of the power amplifier 22. As explained hereinabove, the transformer ratio between the primary winding turns connected in series with the plate 3| of the voltage amplifier tube 2| and the total turns which act as the secondary winding of the-autotransformer connected to the input grid of the following amplifier stage is so chosen that the reflected shunting capacity of the following stage forms with the inductance connected in series with the plate 3i of the voltage amplifier tube 2| a resonant circuit having a resonant frequency in the desired part of the speech frequency range required for a satisfactory hearing aid.

In the amplifier arrangement sion is also 'made for' controllably varying the plate current in the voltage amplifier stage and thereby enable selective control of the characterof the voltage amplification in the way hereinbefore explained in connection with curves of Fig. 12. To this end, the screen grid 33 of the voltage amplifier tube 2| is connected to a tap of a rheostat I06 connected between the lead 43 from the +3 terminal of the amplifier and the lead 49 which in the operating condition of the amplifier is connected by switch 39 to the grounded terminal of the amplifier. By adjusting the tap N of the rheostat I06, the voltage of the screen grid electrode 33 of the voltage amplifier tube 2i may be brought to any desired voltage so as to secure the plate current conditions required for producing the desired character of amplification in the way explained in connection with Fig. 12.

of Fig. 14, provi-.

unit, it may be combined with the battery unit 24. Alternatively, the individual cells of the grid bias battery element 25 may be provided with separate terminal socket members 2 so as to enable compensation for the drop of the plate battery voltage by connecting the cord lead extending from the grid bias terminal C to a lower voltage terminal 2 of the grid bias battery element 26.

Since the usefulness of a hearing aid depends on the degree of its compactness, it is very important to reduce as much as possible the bulk of all the elements of the hearing aid. The output transformer 52 which couples the plate circuit of the power amplifier tube 22 to the receiver 29 is one of the bulky elements of the amplifier. In order to construct an output transformer which has a high impedance on the primary side and is of small size, it is necessary to use high permeability magnetic core laminations. However, since the primary winding of the transformer is connected in the plate circuit of the power amplifier tube 22 and carries D. C. plate current, the core laminations are subjected to saturation and their permeability is reduced. As a result, such transformer requires a larger core than would be required if the problem of core saturation could be eliminated.

In accordance with the invention, the satura tion problem in the amplifier output transformer is eliminated by neutralizing the D. C. plate current in its primary winding by supplying the secondary transformer winding circuit which is traversed by a direct current which exercises on the magnetic core of the transformer an effect substantially equal and opposite to the effect exercised by the D. C. plate current flowing in the primary transformer winding.

As shown in Fig. 14 illustrating such arrangement, the secondary transformer winding 55 of the transformer 52 is connected between the receiver-terminals R of the amplifier unit 23 and t the lead 31 which in the closed position of the cutoff switch 39is connected to the -A terminal of the amplifier, the other receiver terminal RG of the amplifier unit being connected through a lead 5 to the +A terminal of the amplifier unit. With this arrangement, the cutoff switch 39 completes in its closed position a circuit from As explained hereinbefore, wearable hearing aids have to be supplied from a battery unit that is small enough for inconspicuous wear on the body of the user. The voltage of such batteries goes down in use and itis desirable to provide a' convenient way to compensate for the reduction of the amplification resulting from the drop in the voltage of the battery. In the .amplifier arrangement of Fig. 14, this isyaccomplished by providing an additional rheostat ||0 connected across the .amplifier terminals -A and C leading to the C battery 26 and connecting the tap of the rheostat lid to the grid leak resistor 41 which leads to the control grid 34 of the power amplifier tube 22. By actuating the rheostat I In to reduce the negative grid bias when the voltage of the B battery drops down, the operation of the amplifier may be controlled to secure the required amplification notwithstanding the drop of the voltage of the B battery 25. Instead .of placing the grid bias control rheostat H5 in the amplifier the A battery cell 21 to the secondary winding 55 of the output transformer, a D. C. current fiows through the secondary transformer winding 55 in a direction opposite to the D. C. plate current flowing through the primary transformer winding 54.

The drain from the A battery 271 for supplying the D. C. current to the secondary winding 55 is made very small by increasing the number of turns of the actuating winding of the receiver 29 so as to obtain asecondary transformer winding 55 of a large number of turns requiring a drain of only a few milliamperes in order to neutralize the saturating efiect of the D. C. plate current flowing in the primary winding 54 of the output transformer. Alternatively, spurious coupling efi'ects between the magnetic core of the inductance 42 and of the transformer 52 in the construction of Figs. 1 to 5 may be prevented by placing between the cores a magnetic shield, as indicated at MB, of high permeability magnetic material.

The amplifier arrangement described in con- I nection with Figs. 1 to 4 is designed for wear in the vest pocket of the user and for thisfreason it is important that the sound pervious part of the microphone casing is located near the upper end of the casing so that the two shoulders bordering the recessed sound pervious part of the microphone part of the casing are kept open while at the same time eliminating noise caused by the rubbing contact between the clothing of the user and the edges of the microphone holes in the casing wall of the amplifier. shown another way of arranging the amplifier elements in a flat small casing of the size of a cigarette case that is designed to be exposed'to sound, for instance, if worn underneath a sound pervious garment fabric so that its front wall is reached by sound waves propagated in the air. As in the arrangement described in connection with Fig. 5, the various elements of the amplifier unit are fastened to a frame plate 553 which is detachably secured to the curved casing wall. In the arrangement of Figs. to 17, the transformer and the inductance are not located adjacent to each other, but are spaced far apart and the microphone casing is held suspended by its rubber collar in the space between the two transformers. This arrangementmakes it possible to keep the elements which constitute the successive stages of the amplifier spaced apart far enough, thereby reducing the spurious capacitive and magnetic coupling effects that are a source of much trouble in such compact highgain amplifiers. The arrangement of Figs. 15 and 16 makes it also possible to locate the leads of different potential of the amplifier far apart so that their mutual influence is reduced to a minimum.

In Fig. 18 is illustrated how the selective control of the frequency response may be applied to an electron tube hearing aid amplifier arrange= In Figs. 15 to 17 is 1 In the electron tube hearing aid amplifier arrangement which must fit into a compact fiat cas-'.

ing suitable for inconspicuous wear on the body of the user as described hereinabove in connection with the specific exemplifications thereof, a gain of more than 100 over the frequency range between about 500 and 4000 cycles and of at least 200 in a narrower part of the speech frequency range is obtained with a single voltage amplification stage by using as a coupling impedance for the voltage amplifying space discharge tube a coupling unit having an overall volume of the order of about one cubic inch or less formed of magnetic core laminations having a cross section of aboutf of one square inch or less and suflicient windings so that it has a coupling impedance of at least one megohm over at least one octave of the speech frequency range between about 500 cycles and 4000 cycles per second while keeping the D. C. anode current between the limits of about 5 microamperes and 2 milliamperes at the maximum available voltage. The voltage amplifier stage is also associated with means for adlusting the direct current in the anode circuit to a selected value in order to secure the character of amplification most suitable for compensating the hearing defects of the user. A feature of the invention resides in the utilization of the inductment using two resistance coupled voltage amplifier stages in lieu of pled voltage amplifier tion with the hearing of Fig. 5.

As shown in Fig. 18, it comprises a microphone 20 and a receiver are with a battery assembly 20, 25, 2'1? similar to those shown in Fig. 5 and an amplifier formed of two resistance coupled volt= age amplification stages associated with the electron amplifier tubes 230, 223i and a power amplifier stage associated with a tube 22 which is similar to the power amplifier stage shown in Fig. 5. The voltage amplifier tubes 231i, 236 ar shown as pentode tubes, each having a plate 3!, a suppressor grid 32, a screen grid 83, a cointrol grid 3d and a cathode 35 which are interconnected to the battery assembly similar as in Fig. 5. The output of the microphone is impressed on the control grid of the first tube 239 across a resistor 235 and the output of the arm plifiel tube 238 is impressed across a coupling resistor 2353 upon the control grid of the second amplifier tube 23L The amplified output of the second amplifier tube 230 .is impressed across the coupling resistor 235 on the control grid of thepower amplifier through the output ceiver 29.

By connecting in shunt to the output transformer 52 the condenser 53 and a volume control rheostat 54 arranged and proportioned in the way described in connection with Fig. 5, the frequency response may be controlled to deliver to the rece'iver an output which is substantially fiat over the frequency range between 500 cycles and 4000 cycles or to selectively boost the low frequency the single inductively coustage described in oorlnecaid amplifier arrangement transformer 52 to the reresponse and reduce the high frequency response as described hereinbefore.

22 which delivers its output ance of the receiver coupled to the output circuit of the power amplifier stage in a circuit portion connected parallel to the output circuit to form therewith a. resonant circuit having a resonant frequency at a selected point of the speech frequency range, and selectively controlling the effectiveness oi the capacitance so as to compensate for the hearing defects of the user.

Spurious capacitive and magnetic coupling effects between the elements of the compact high gain amplifier unit are suppressed by the special distribution of the various elements within the compact casing of the hearing aid amplifier.

Additional reduction of the bulk of the ampliher is made possible by neutralizing the effect of the D. C. plate current flowing in the primary winding of the output transformer, thereby enabling a substantial reduction of the bulk of the transformer.

By employing only a single main plug for establishing detachable connections from batteries and the receiver to the amplifier unit and providing detachable plug connections between the terminal members of the main plug and the ends of th receiver cord leads the user is able to exchange the receiver cord which is subjected to the greatest amount of wear without disturbing the main plug and its battery cord.

Various other modifications of the invention 1'. In an inconspicuously-wearable hearing aid:

a mechano-electric transducer operating as a microphone, such as a crystal-microphone; an electro-mechanical transducer operating as a receiver; a source of direct-current voltage; and an electron tube amplifier having only one voltage amplifier stage and only one power amplifier stage interconnected between said microphone, said receiver and said source; a fiat small casing suitable for substantially of the user and enclosing said microphone'and hidden wear on the body substantially all the elements of said amplifier so that the microphone shall be actuated by propagated sound; said voltage amplifier stage having a multi-electrode amplifier tube including an anode, and an interstage coupling inductance connected between said anode and said source and having a magnetic core of a cross section of the order of about 6 of one square inch or less; the

elements of said inductance and of said voltage amplifier stage being so designed and correlated, and their operating voltages being so set, -that D. C. anode currents through said inductance are confined between values of the order of about ten microamperes and two milliamperes at which it operates with peaked frequency response characteristics and acts as a coupling impedance of the order of about one megohm or more over about two octaves or more of the speech frequency range between about 500 and 4000 cycles per second; the elements of said amplifier and of said transducers being so designed and interconnected and their characteristics being so correlated that the frequency response characteristics of at least one of said transducers compensates for the peaked response characteristics of said coupling inductance for securing a receiver output of a magnitude and a frequency response required from an electron tube amplifier hearing aid.

2. In aid: a mechano-electric transducer operating as a microphone, such as a crystal microphone; an electro-mechanical transducer operating as a receiver; a source of direct-current voltage; and an electron tube amplifier having only one voltage amplifier stage and only one power amplifier stage interconnected between said microphone, said receiver and said source; a flat small casing suitable for substantially hidden wear on the body of the user and enclosing said microphone and substantially all the elements of said amplifier so that the microphone shall be actuated by the other circuit elements of an inconspicuously-wearable hearing propagated sound; said voltage amplifier stage 7 having a multi-electrode amplifier tube including an anode, and an interstage coupling inductance connected between said anode and said source and having an overall volume of the order of about one cubic inch or less and a magnetic core of a cross section of the order of about of one square inch or less; the elements of said inductance and of said voltage amplifier stage being so designed and correlated, and their operating voltages being so set, that D. C. anode currents through said inductance are confined between values of the order of about five microamperes and one milliampere at which it operates with peaked frequency response characteristics and acts as a coupling impedance of the order of about two megohms or more for securing a peaked stage gain ratio of the order of 100 or more over about one octave or more-of the speech frequency range between about 500 and 4000 cycles per second; the elements of said amplifier and of said transducers being so designed and interconnected and .their characteristics being so correlated that the frequency response characteristics of at least one of said transducers compensates for the peaked response characteristics of said coupling inductance for securing a receiver output of a magnitude and a frequency response required from an electron tube amplifier hearing aid.

an inductance of substantial magnitude; a source of direct-current voltage; and an electron tube amplifier including a voltage amplifier stage and a power amplifier stage interconnected between said microphone, said receiver and said source; a fiat small casing suitable for substantially hidden wear on the body of the user and enclosing said microphone and substantially all the elements of said amplifier so that the microphone shall be actuated by propagated sound and cause said amplifier to deliver to said receiver an amplified microphone input; said power amplifier stage having an amplifier tube and an output circuit coupling said tube to said receiver winding for delivering the amplifier output to said receiver; and a circuit portion connected parallel to said output circuit and including. a capacitance constituting with the inductance of said receiver reflected into'said output circuit a substantially resonant circuit at a predetermined frequency-range part between about 500 and 4000 cycles per second; the elements of said circuit portion being so designed and correlated to said amplifier that when the capacitance of said circuit portion is rendered ineffective, said amplifier operates with a predetermined maximum gain Within said frequency-range part and that when said capacitance is rendered efiective, said amplifier operates with a materially higher gain than said predetermined maximum gain within said frequency-range part for enabling controllable setting of the overall frequency response of the receiver output in accordance with the needs of the user. I

4. In an inconspicuously-wearable hearing aid:

-a self-generating microphone, such as a crystal microphone, having an actuating diaphragm; a receiver including an actuating winding having an inductance of substantial magnitude; a source of direct-current voltage; and an electron tube amplifier including a voltage amplifier stage and a power amplifier stage interconnected between said microphone, said receiver and said source; a fiat small casing suitable for substantially hidden wear on the body of the user and enclosing said microphone and substantially all the elements of said amplifier so that the microphone shall be actuated by propagated sound and cause said amplifier to deliver to said receiver an amplified microphone input; said power amplie fier stage having an amplifier tube and an output circuit including a transformer coupling said tube to said receiver winding for delivering the amplifier output to said receiver; and a circuit portion connected parallel to the primary side of said transformer and including a capacitance constituting with the inductance of said receiver reflected into said output circuit a substantially resonant circuit at a predetermined frequencyrange part between about 500 and 4000 cycles 'persecond; the elements of said circuit portion being '*so designed and correlated to the other circuit elements of said amplifier that when the capacitance of said circuit portion is rendered ineffective, said amplifier operates with a predetermined maximum gain within said frequencyrange part and that when said capacitance is rendered effective, said amplifier operates witha materially highergain than said predetermined maximum gain within said frequency-range part; and means associated with said circuit portion for controllably rendering said capacitance ineffective so as to set the overall frequency response of the receiver output in accordance with the needs of the user.

\5. A hearing aid as defined by claim 1 in which the response characteristics of the mechanoelectric transducer operating as a receiver compensates for the peaked response characteristics of the coupling inductance in a portion of the speech frequency range above the peak-frequency part of the peaked response or the coupling in- .ductance.

inductance.

HARRY B. SHAPIRO. 

